Beyond epithelial‐to‐mesenchymal transition: Common suppression of differentiation programs underlies epithelial barrier dysfunction in mild, moderate, and severe asthma

Abstract

Background Epithelial barrier dysfunction is a central feature in the pathogenesis of allergic disease. Epithelial-to-mesenchymal transition (EMT) has been proposed as one mechanism afflicting barrier in asthma. However, genes and pathways involved in aberrant epithelial-mesenchymal signaling, and their relationship to asthma severity, are poorly understood. Methods We used unbiased gene network analysis to evaluate functional convergence in epithelial gene expression signatures across multiple public access transcriptomics datasets of human asthma, followed by text mining to evaluate functional marker relevance of discovered genes. We objectively confirmed these findings in epithelial brushings and primary asthmatic epithelial cells cultured in different biological contexts. Results We found a striking suppression of epithelial differentiation in asthma, overrepresented by insufficiency in insulin and Notch signaling, but with the absence of conventional EMT markers. We identified EFNB2, FGFR1, FGFR2, INSR, IRS2, NOTCH2, TLE1, and NTRK2 as novel markers central to dysregulation of epithelial-mesenchymal signaling, but surprisingly overlooked in asthma research. We found that this “core” signature of asthma is shared by mild, moderate, and severe forms of disease, progressing with severity. Loss of epithelial differentiation induced by insulin deprivation in normal human bronchial epithelial cells cultured in organotypic conditions closely approximated gene expression in asthmatic epithelial brushings. Conclusions The comparative analysis of publically available transcriptomes demonstrated that epithelial barrier dysfunction in asthma is characterized by persistent underlying de-differentiation program with complex etiology. The lasting alteration of the asthmatic epithelial cell transcriptome implicates regulation involving metabolism and epigenetics, beyond EMT driven by injury and repair in chronic inflammation.

Epithelial barrier repair and prevention of allergy

“…Epithelial-mesenchymal transition facilitates the development of tissue fibrosis in response to injury and chronic inflammation (185). Although not described in AD, it plays an important role in asthma (186,187) and EoE (188).…”

Shaping eosinophil identity in the tissue contexts of development, homeostasis, and disease

“…For example, normal mammary gland development has been described as wound‐like in both mice and humans . Similarly, morphogenesis of the lung continues during postnatal development, characterized by the deposition of the provisional extracellular matrix, increased mesenchymal–epithelial communication, differentiation of mature stratified epithelium, as well as processes associated with the loss of epithelial differentiation in chronic allergic disease . The only apparent difference between normal and pathological states of the tissue undergoing remodeling is in heterochrony; timing of the onset, offset, and persistence of these signals.…”

“…The LIAR hypothesis postulates that eosinophils are attracted to sites of high stem cell activity that accompany all tissue generation and regeneration events. The epithelial–mesenchymal transition (EMT) has recently been implicated in multiple allergic diseases as a mechanism causative of mucosal barrier disruption . In homeostatic conditions, EMT occurs as a mechanism of maintaining epithelial barrier integrity by potentiating the mesenchymal unit to create new layers of differentiated epithelial cells .…”

“…1 16,17 Similarly, morphogenesis of the lung continues during postnatal development, characterized by the deposition of the provisional extracellular matrix, increased mesenchymal-epithelial communication, differentiation of mature stratified epithelium, 18,19 as well as processes associated with the loss of epithelial differentiation in chronic allergic disease. 20,21 The only apparent difference between normal and pathological states of the tissue undergoing remodeling is in heterochrony; timing of the onset, offset, and persistence of these signals. In normal development and injury repair, homeostatic modeling programs are perfectly timed, while in disease they are perpetuated, perhaps associated with a continuous attempt to return tissue to a homeostatic state.…”

“…The epithelial-mesenchymal transition (EMT) has recently been implicated in multiple allergic diseases as a mechanism causative of mucosal barrier disruption. 20,22,23 In homeostatic conditions, EMT occurs as a mechanism of maintaining epithelial barrier integrity by potentiating the mesenchymal unit to create new layers of differentiated epithelial cells. 18 At sites such as the intestine, epithelial shedding rates are high and mesenchymalepithelial turnover is much higher.…”